1. Field of the Invention
The present invention relates to an apparatus for the application of a curable composition to a fastener, especially to a threaded fastener. In particular, the invention relates to an apparatus for the application of an at least two stage curable composition to a fastener. The invention further relates to curable compositions for application to a fastener, particularly by the use of the said apparatus. In particular the invention relates to adhesive compositions which are suitable for thread locking or thread sealing applications. In general the present invention relates to a polymerisable composition for the sealing and locking of engineering parts, particularly interfitting parts such as fasteners, more particularly threaded fasteners such as nuts, bolts, screws, threaded nails and the like, or parts such as pipe joints which may be threaded or slip-fitted. Compositions of the invention may be used to assemble engineering parts where close tolerances are designed between adjacent surfaces of the parts. The invention particularly relates to pre-applied compositions.
2. Brief Description of Related Technology
It is known to provide an apparatus for applying material to fasteners, in particular, threaded fasteners. One of the problems which have been associated with conventional apparatus for applying a composition includes the problem of contact marks. Contact marks arise where fasteners touch off each other during the coating process. If the coating is not dry to touch then some of the coating may come off onto another fastener, leaving a mark where contact was made (on one or both fasteners). Such marks are known as contact marks.
One of the reasons for eliminating contact mark problems includes improving the appearance of the end product. In particular, it is desirable that fasteners will have composition only where it is required and will not have small amounts of the composition on other parts of the fastener, which detract from the aesthetic appearance of the fasteners.
Having composition on undesired parts of the fastener can also deleteriously affect the handling of the fasteners by automated machines (for example, robotic machines such as robotic grippers and the like). Due to the continued automation of assembly lines and the like, it is desirable that fasteners can be handled by automated machines, for example, machines for placing the fastener in a fastening position, and/or fastening parts mechanically with the fasteners. Such automated apparatus will normally be designed to grip the fastener at a part of the fastener which has not had composition applied. This is to ensure that composition does not find its way onto the handling apparatus for the fastener. However, if composition has made its way onto other parts of the fastener (as discussed above), then it tends to come off on the handling apparatus, eventually causing problems with gripping of fasteners. It may interfere with moving parts of at least that part of the apparatus which grips the fastener and the like. Other methods of handling fasteners include pneumatic lines (often referred to as “shoots”) which essentially are hollow tubes through which the fasteners are blown by compressed air pressure. The fasteners can clog up in the tubes if the interior of the tubes becomes soiled with composition due to contact of the fastener with the tube (contact marks on the interior of the tube). Accordingly it is desirable that any coating applied is substantially dry to touch when being passed through such systems.
Certain forms of apparatus which have been provided for applying coatings to fasteners include a drying oven in the form of a high temperature oven which is used to dry the composition. There are inefficiencies in the process, as the amount of energy required to dry the composition is high. Furthermore, batching the fasteners together for drying in the oven can lead to contact marks, as discussed above. Evaporation of water and/or solvent from a composition by heating may create contaminated extracted air which is another potential disadvantage.
It is desirable, therefore, to provide a handling mechanism that can carry components through both the coating and drying processes to create a dry-to-touch composition applied to the fastener without creating an opportunity for contact marks to be formed by touching (e.g. by collision) of fasteners. It is also desirable to provide a compact apparatus for application of curable composition. It is also desirable to provide an apparatus, which can have high throughput of components, which are coated, suitably in a compact arrangement.
One such handling apparatus is described in U.S. Pat. No. 6,027,568 (Wallace et al). Wallace et al. describe an apparatus and method for providing a masking, insulating and/lubricating barrier coating on a portion of threads of fasteners. The apparatus employs applicator guns, which fire a jet of composition onto the fasteners. The fasteners are conveyed along the apparatus by a conveyor and are delivered to an oven where they are dried. Accordingly, it is likely that contact marks would be formed in batching of the fasteners for drying in an oven. The compositions applied are not curable.
UK Patent Application No. GB 2 255 781 (Reactive Industries, Inc.) describes a composition which is UV-curable and which is applied to fasteners by an apparatus. The apparatus includes a conveyor having two spaced apart belts for conveying fasteners from a loading station past applicators and a heating station to a UV station. The fasteners are caught between the two belts with two opposing sides at the head of the fastener resting on the top of the belts and the stem of the fastener depending vertically downwards in which position the fasteners are held throughout the processing steps.
UK Patent Application No. GB 2 255 781 (Reactive Industries, Inc.) already mentioned above describes a composition comprising a hardener-containing component which includes (a) a hardener capable of polymerising a resin, e.g., an epoxy resin; (b) a UV curable film-forming compound comprising a dimethacrylate; and (c) a photoinitiator compound capable upon exposure to UV light of curing (b) to form an integral, thin continuous, dry, substantially pin-hole free, flexible, non-tacky outer protective skin. The hardener containing component is preferably fluid and microencapsulated and can be used in an adhesive system additionally comprising a resin-containing component which includes (1) a resin, e.g., an uncured epoxy resin which is polymerisable by the hardener component, (2) a UV-curable film-forming compound, e.g., a dimethacrylate; and (3) a photoinitiator capable upon exposure to UV light of curing the film-forming compound (2) to form an integral non-tacky outer protective skin. The resin component may be micro-encapsulated. In this system, also, the UV curing forms only an outer protective skin and the components for the second polymerization are free to move beneath the skin or, in the event of rupture of the skin, to escape from it. The film-forming compounds used are monomers, particularly dimethacrylate monomers, and there is no suggestion of use of oligomers. The rheology of the composition is not discussed and, in particular, the use of a thickener is not mentioned.
Compositions which are suitable for use in threadlocking applications are known. Such compositions find use in many applications. In general the compositions are used as described above to lock interengaging threads together. The composition is generally placed on one or both of components which have reciprocal interengaging screw threads. When the components are screwed together using the screw threads the composition acts to lock the threads in the desired interengaged position.
In general the applications in which compositions of the present invention are used include any application where it is desired to lock screwthreads together so that the components which are screwed together do not become disengaged unintentionally, for example because of vibrational or other forces. Such applications include use in the construction of automobiles etc.
Compositions have been used which must be applied at the time of threading one component into another, as the compositions remain substantially liquid before cure. Such compositions include anaerobically curable compositions.
A pre-applied composition is described in U.S. Pat. No. 4,059,136, where two physically separated deposits are applied to the threaded part. A cover coating is then applied over the deposits.
European Patent No. 0 077 659 (Thompson) describes a pre-applied polymerisable fluid for sealing and locking engineering parts. The composition has two mechanisms for curing and two curing reactions take place. The first mechanism is a UV light cure. An opacifier is dispersed in the fluid so that the fluid becomes substantially opaque to radiation. After the fluid is applied to the component it is exposed to UV radiation whereupon a coating is formed, creating a surface layer which is a dry, tack-free crust. The subcutaneous fluid is unaffected by the radiation and remains in a generally liquid state. When the component is threaded into another the surface layer breaks and the second polymerisation (such as a free radical addition polymerization ) is initiated and the second cure reaction takes place. The second polymerisation mechanism acts to lock the threads together.
In Thompson, only a skin is formed in the first polymerization and the remainder of the composition remains fluid below the skin. There is a risk therefore that during handling of the coated engineering parts the skin may be disrupted and the fluid composition may leak out.
European Patent No. 0 548 369 (Usami) describes a pre-applied adhesive composition for application to the threaded contact faces of a screw member such as a screw. The composition comprises a photo-hardening binder in which a secondary curable composition is dispersed. The secondary curable composition includes microencapsulated reactive monomer/activator/initiator. Exemplified compositions include those containing (i) microencapsulated bisphenol A dimethacrylate and a radical generator; (ii) microencapsulated bisphenol A dimethacrylate and DMPT; and (iii) Three Bond product no. 3057D. The only information given about the product 3057D is that it is a UV-hardening acrylic resin containing a photoinitiator. Other similar compositions are disclosed in the other “Embodiments” described. In two “Embodiments” bisphenol A epoxy resin is encapsulated. In one of these “Embodiments”, the microencapsules enclosing bisphenol A epoxy resin are used with Three Bond product no. 3057D, salicyclic acid derivative and aromatic diamine. The present applicants have not been able to obtain a sample of Three Bond product 3057D and therefore have no information about the acrylic resin contained in it. In the other “Embodiment”, the microcapsules enclosing bisphenol A epoxy resin are used with dialkylamino-acrylamide and a photoinitiator (DAROCURE 1173). There is a general indication in the description that a filler may be added but no filler is used in the “Embodiments”. There is no suggestion of using a thickener, nor is there any enabling teaching about the acrylic resin. So far as the present applicants are aware, no product has been commercialised under the European Patent No. EP 0 548 369.
Notwithstanding the various compositions that have been provided it is desirable to provide an alternative composition which is suitable for use in thread sealing/thread locking applications. It is desirable that such compositions are suitable for pre-application to threaded articles, for example to bolts, for later use. It is desired that the composition can be applied to form a dry-to-touch product on the threaded article. It is also desirable that the composition can be later activated to lock threaded articles together with desirable properties of the bond formed between the threaded articles. To achieve s this the composition must be storage stable before application and then remain stable for subsequent use.